Automatic merge from submit-queue

Use ubuntu-slim to reduce size of the iperf:e2e image

from
```
gcr.io/google_containers/iperf     e2e      8b3cc7064090        5 weeks ago     737.9 MB
```
to
```
gcr.io/google_containers/iperf     e2e      204325491636        33 seconds ago  61.09 MB
```

related to https://github.com/kubernetes/kubernetes/pull/25784#issuecomment-221706886
ping @bprashanth